In communication networks which applies LTE (Long Term Evolution) based radio access technologies, accurate time and phase alignment of the internal clock is important. Time and phase synchronisation is required for LTE-TDD (Time Division Duplex) many LTE-FDD (Frequency Division Duplex) coordination features e.g. for joint transmission, a wireless communication device receives data from multiple cells or multiple RBSs (Radio Base Stations), which offers better performance, but puts harder requirements on synchronisation. In packet synchronisation networks, a major problem for synchronisation protocols is the variance in the send time, access time, propagation time, and the receive time.
From the infrastructure perspective, mobile operators have a broad range of topologies to support. The physical network using different technologies such as microwave, fibre and copper wire will enable/limit different capabilities and characteristics. These differences in physical transport and in the different types of topologies, creates delay and delay variation that is unpredictable.
One solution for synchronising internal clocks in communication network is to distribute PTP (Precision Time Protocol) messages from a Grandmaster entity, which in generally is located centralised in the communication network, to PTP-clients at each cell site. The PTP protocol distributes PTP messages from a Grandmaster entity to transport network nodes and access network nodes who update their internal clocks based on the received time information in order to stay synchronised. A PTP system is a distributed, networked system consisting of a combination of PTP and non-PTP devices. PTP systems include a grandmaster entity, boundary clock nodes, ordinary clock nodes, and transparent clock nodes. The grandmaster entity is a form of synchronisation master node. Often the Grandmaster entity is located in a centralized part of the network; causing PTP messages to travel multiple hops. A “boundary clock” has multiple network connections and can accurately bridge synchronisation from one network segment to another. A synchronisation master is selected for each of the network segments in the system. The root timing reference is called the Grandmaster clock. The Grandmaster entity transmits synchronisation information to the clocks that are in its network segment. The boundary clocks with a presence on that segment then relay accurate time to the other segments to which they are equally connected. The transparent clock modifies PTP messages by including appropriate timestamps as they pass through the device. The Timestamps in the PTP messages are compensated for time spent traversing the network and equipment e.g. (switch/router).
With reference to FIG. 1, which is a schematic overview, a scenario of a communication network will now be described according to one example.
The communication network comprises a transport network with a plurality of transport network nodes 200, e.g. suitable switches, routers or gateways. In the FIG. 1, is further illustrated, a synchronisation master node 230, a radio base station 220 of a suitable radio access technology, and a boundary clock node 250. Within this disclosure, as well synchronisation master nodes 230, radio base stations 220 (which typically utilises an ordinary clock) and boundary clock nodes 250 will be referred to as transport network nodes too. In the figure a mobile telephone 240 is illustrated which communicates via an access network with the radio base station 220, in accordance with any suitable radio access technology, e.g. LTE (Long Term Evolution), LTE-Evolution, 5G (of 3GPP (Third Generation Partnership Program)), UMTS (Universal Mobile Technology System), or HSPA (High Speed Packet Access).
The synchronisation master node 230 produces a timing reference which is delivered by synchronisation packets to the ordinary clock nodes 220 or boundary clock nodes 250 along respective paths (marked with dashed lines and dash-dotted lines respectively).
With reference to FIG. 2, which is a schematic signalling diagram, a scenario related to synchronisation will now be described according to one example.
The synchronisation master node sends a PTP packet which comprises a SYNC message and a time t1 towards a boundary clock node or an ordinary clock node possibly along a path of transparent clock nodes. The PTP packet arrives at the boundary clock node at a time t2. The time t2 depends on the propagation time along the path but also on various delays caused by the transport network nodes (e.g. buffering and queues). In response to the received SYNC message the boundary clock node sends a DELAY_REQ message in a PTP packet towards the synchronisation master node at the time t3. Also this PTP packet is delayed and arrives to the synchronisation master node at the time t4. The time t4 is then sent together with a sequence identifier with a DELAY_RESP message in a PTP packet to the boundary clock node or ordinary clock node. Based on the times t1, t2, t3 and the arrival t4 the boundary clock node or the ordinary clock node estimates a time offset towards the synchronisation master node as ((t2−t1)+(t4−t3))/2.
It is desired to enable network operators to make better use of installed communication resources, e.g. in order to serve end-users more appropriately.